string calculator

stringcalculator.rb

module StringCalculator
  extend self

  DELIMITERS = ["\n", ","]
  DELIMITER_MARK = '//'

  def add(string)
    numbers = extract_numbers(string)
    assert_not_negatives(numbers) 
    total(numbers)
  end

  private

  def extract_numbers(string)
    string.split(delimiters(string)).map(&:to_i)
  end

  def delimiters(string)
    Regexp.union(delimiters_regexp(string))
  end

  def delimiters_regexp(string)
    regexp = DELIMITERS.tap do |delimiters|
      delimiters << string[DELIMITER_MARK.length] if string.start_with?(DELIMITER_MARK)
    end
  end

  def assert_not_negatives(numbers)
    negatives = numbers.select { |number| number < 0}
    raise "negatives are not allowed: #{negatives.join(', ')}" if negatives.size > 0
  end

  def total(numbers)
    numbers.inject(0, :+)
  end
end

describe StringCalculator do
  context 'empty string' do
    it 'returns 0 for ""' do
      subject.add('').should eq(0)
    end
  end

  context 'simple number' do
    it 'returns 1 for "1"' do
      subject.add('1').should eq(1)
    end

    it 'returns 2 for "2"' do
      subject.add('2').should eq(2)
    end
  end

  context 'two numbers' do
    it 'returns 3 for "1,2"' do
      subject.add('1,2').should eq(3)
    end

    it 'returns 5 for "2,3"' do
      subject.add('2,3').should eq(5)
    end

    it 'returns 12 for "10,2"' do
      subject.add('10,2').should eq(12)
    end
  end

  context 'three numbers' do
    it 'returns 3 for "1,1,1"' do
      subject.add('1,1,1').should eq(3)
    end
  end

  context 'with line break' do
    it 'returns 5 for "2\n3"' do
      subject.add("2\n3").should eq(5)
    end

    it 'returns 9 for "3\n3,3"' do
      subject.add("3\n3,3").should eq(9)
    end
  end

  context 'with custom delimiter' do
    it 'returns 10 for "//;\n5;5"' do
      subject.add("//;\n5;5").should eq(10)
    end
  end

  context 'with negative numbers' do
    it 'throws error with the list of the negative numbers' do
      expect { subject.add('-1,-2,-3') }.to raise_exception(
        'negatives are not allowed: -1, -2, -3')
    end
  end

  it 'integration' do
    subject.add("//;\n5;5\n2,3,300\n1000").should eq(1315)
  end
end

I did some improvements :-)

Couple of improvements and simplifications. https://gist.github.com/pasku/90f9138bdb37af5dc99b

@pasku I have applied some of your improvements here :-)